The strain differences of heart and aorta weight in fifteen strains of rats were investigated for characterizing the cardiac hypertrophy from the genetic aspect. A significant strain differnce in the heart weight was observed at ten weeks of age, and the degree of genetic determination was 67.0 plus or minus 5.8 percent. The distribution of heart weight was divided into hypertensive and normotensive groups. In the hypertensive group, the heart weight increased in proportion to the blood pressure levels. On the other hand, there was no relationship between heart weight and blood pressure level in the normotensive group in spite of large strain difference of heart weight. These data indicated that the cardiac hypertrophy in rat is regulated by genetic factors in addition to blood pressure. A similar result was obtained with the aortic weight. However, the effect of genetic factors was less important for aorta weight than for heart weight.